Element for wavelength conversion and/or optical computing

ABSTRACT

When forming a periodically-poled structure on a nonlinear optical crystal  1  that permits wavelength conversion and/or optical computing, the group velocity matching conditions are determined to synchronize the group velocity of the incident light L 1  with that of the outgoing light L 2 , and the polarization reversal period of the periodically-poled structure is determined to satisfy quasi-phase matching conditions for the aforementioned wavelength conversion and/or optical computing. As a result, the problems associated with wavelength conversion of the pulsed light due to a difference in the group velocity are suppressed.

TECHNICAL FIELD

The present invention belongs to the technical field of a nonlinearoptical crystal and particularly relates to an element for wavelengthconversion and/or optical computing, which is equipped with aperiodically-poled structure for wavelength conversion or opticalcomputing by a quasi-phase matching method.

BACKGROUND ART

Nonlinear optical crystals such as LiNbO₃, LiTaO₃ and the like have beenpreferably used as materials of elements used for wavelength conversionsuch as second harmonic generation (SHG), optical parametricoscillation, optical parametric generation (including amplification),difference frequency generation, sum-frequency generation and the like.

As a means for satisfying the phase matching conditions for suchwavelength conversion, quasi-phase matching (QPM) including formation ofa periodically-poled structure (hereinafter to be also referred to as a“poled structure”) on a nonlinear optical crystal has been activelyconducted in recent years. The quasi-phase matching is described indetail in, for example, a publication, Optical Second HarmonicGeneration and Polarization Reversal, Kurimura, Solid-State Physics,29(1994) (75–82) and the like.

As shown in FIG. 2, a poled structure element (wavelength conversionelement) is an element wherein the polarizational direction (z directionin the Figure) of a nonlinear optical crystal 10 is periodicallyreversed (i.e., nonlinear optical constant has been modulated) toachieve a high wavelength conversion efficiency, and the nonlinearoptical constant to be utilized is exclusively d₃₃, because its value isthe highest. That the nonlinear optical constant d₃₃ can be used is theadvantageous aspect of the quasi-phase matching method.

In conventional poled structure elements, what is called a z plate(crystal substrate processed to make the substrate surface perpendicularto the z-axis of optical crystal) is used and a polarization reversalperiod utilizing the nonlinear optical constant d₃₃ is formed. As shownin FIG. 2, the polarized light direction of an incident light L10 andthe polarized light direction of a wavelength converted outgoing lightL20 are both parallel to the z-axis of the nonlinear optical crystal. Inthis way, only the utilization of the nonlinear optical constant d₃₃ hasbeen conventionally taken note of and group velocity matching of theincident light and the outgoing light has not been considered at all.

For wavelength conversion, a light having a pulse train (pulsed light)is sometimes used as an incident light. Examples thereof includeconversion of, a pulsed light having a wavelength of 1.5 μm to a pulsedlight having a wavelength of 0.78 μm by SHG, computing (e.g., samplingand gating for time-division multiplex communication, channel conversionand routing for wavelength multiplex communication) of a pulsed lighthaving a wavelength of 1.5 μm and a pulsed light having a wavelength of0.78 μm, and the like.

However, when the present inventors studied wavelength conversionbehavior of the above-mentioned conventional poled structure element, itwas found that, when a pulsed light is handled, the incident light andthe outgoing light are separated in space and in time, along with thepropagation of the light, due to a difference in the group velocitybetween the incident light and the outgoing light, and as a result, thefollowing various problems such as those described below occur.

For wavelength conversion of continuous light, for example, sinceincident light exists over the entire length of the element, theconversion efficiency and computing efficiency are expected to beimproved by prolongation of the element length. In contrast, when ashort pulsed light is to be handled, such as pulse-number 1 Tbit/sec orabove (=pulse width 1 ps or below), the incident light and the outgoinglight are separated due to a difference in the group velocity betweenthem, posing a problem in that the conversion efficiency and computingefficiency are not improved even if the element length is prolonged. Aproblem also occurs in that, as a result of wavelength conversion, thepulse width of the outgoing light is extended depending on thedifference in the group velocity, making retention of the pulse shapedifficult, and accurate computing results cannot be obtained. In somecases, a problem also occurs in that pulses before and behind in thepulse train interfere with each other and produce serious errors incomputing. Such problems are clearly recognized when the pulse width isshorter and the pulse-number is higher.

In addition, since this difference in the group velocity depends on thewavelength of the incident light and the wavelength of the outgoinglight (converted light), it becomes a factor that limits the wavelengthband of the incident light. In a 1.5 μm band wavelength variable lightsource using 0.78 μm wavelength as an exciting-light source, moreover,the realizable wavelength band is limited due to the group velocitydispersion.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an elementfor wavelength conversion and/or optical computing, wherein theabove-mentioned problems associated with wavelength conversion of thepulsed light are suppressed.

Accordingly, the present invention is characterized by the following.

(1) An element for wavelength conversion or optical computing, which isa poled structure element comprising a periodically-poled structureformed on a nonlinear optical crystal, thereby to convert an incidentlight to an outgoing light by wavelength conversion and/or opticalcomputing, wherein group velocity matching conditions are determined tosynchronize the group velocity of the incident light with that of theoutgoing light, and a polarization reversal period of the aforementionedperiodically-poled structure has been determined to satisfy quasi-phasematching conditions for the aforementioned wavelength conversion and/oroptical computing.(2) The element of the above-mentioned (1), wherein the nonlinearoptical crystal is MgO doped LiNbO₃.(3) The element of the above-mentioned (1), wherein a nonlinear opticalconstant of the nonlinear optical crystal used to satisfy theabove-mentioned quasi-phase matching conditions is an off-diagonalcomponent of d tensor.(4) The element of the above-mentioned (3), wherein the above-mentionedoff-diagonal component of the d tensor is d₃₁.(5) The element of the above-mentioned (1), wherein the wavelengthconversion and/or optical computing are/is performed by second harmonicgeneration, optical parametric oscillation, optical parametricgeneration, sum-frequency generation or difference frequency generation.(6) The element of the above-mentioned (1), wherein the incident lightis in a pulse train having a pulse width of 1 ps or below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the poled structure element of thepresent invention, wherein 1 is a nonlinear optical crystal, L1 is anincident light and L2 is an outgoing light.

FIG. 2 is a schematic view showing a conventional poled structureelement.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, an element for wavelength conversion and/or opticalcomputing according to the present invention (hereinafter to be referredto as “the element” for explanation) is an element comprising anonlinear optical crystal 1 and the poled structure formed on thecrystal (in this Figure, a poled structure is formed on the entirety ofthe crystal). The significant characteristics here are the facts thatthe group velocity matching conditions are determined to synchronize thegroup velocity of an incident light L1 with that of an outgoing lightL2, and a polarization reversal period of the aforementionedperiodically-poled structure is determined to satisfy quasi-phasematching conditions for the aforementioned wavelength conversion and/oroptical computing.

By constituting the element as mentioned above and quasi-phase matchingwhile matching the group velocity between the incident light (e.g.,wavelength 1.5 μm) and the outgoing light (e.g., wavelength 0.78 μm),sufficient conversion efficiency can be realized even when anultra-short (ca. 100 fs) pulse laser beam is used as the incident light.

As described under the Background Art, the most significantcharacteristic of conventional poled structures is the use of d₃₃, themaximum value of the nonlinear optical constant (nonlinear opticaltensor component). However, such an element does not take into accountthe group velocity matching. Thus, when a short-pulsed light is handled,the incident light pulse and the outgoing light pulse are separated soondue to the difference in the group velocity. Consequently, the distancebefore separation, or the length of the element effectively contributingto the wavelength conversion and/or optical computing, is only about 1mm.

In contrast, in the present invention, since group velocity matchingconditions are met and quasi-phase matching is provided utilizing anoff-diagonal component of the nonlinear optical constant, even in thecase of a short pulse or when the length of the element is prolonged,the incident light pulse and the outgoing light pulse are not separatedover the entire length, thus contributing to the wavelength conversionand/or optical computing, which in turn increases the conversionefficiency.

Furthermore, since the element of the present invention satisfies thegroup velocity matching conditions, the wavelength band is dramaticallybroadened for, for example, a 1.5 μm band wavelength variable lightsource using wavelength 0.78 μm as an exciting-light source, due to thegroup velocity matching. Namely, when a light near 1.5 μm is to begenerated by an optical parametric effect (e.g., optical parametricgeneration: OPG) using an exciting light (0.78 μm), the wavelengththereof is variable by changing the wavelength of the exciting light,crystal temperature and polarization reversal period, wherein a broadband wavelength variable 1.5 μm band light source can be realized usinga wavelength 0.78 μm as an exciting-light source.

In addition, the element of the present invention can afford wavelengthconversion and/or optical computing by quasi-phase matching withoutbreaking the pulse shape. For optical communication and the like, a 1.5μm light having a narrow pulse width (1 ps or below) is necessary. Inthe absence of group velocity matching between the exciting light (0.78μm) and the generated light (1.5 μm), as in conventional cases,short-pulsing of an exciting light does not result in a short pulsewidth of the generated light, but causes poor conversion efficiency. Byapplying the present invention, a wavelength variable light sourceaffording a short pulse width of the generated light and high conversionefficiency can be realized.

By determining the group velocity matching conditions to synchronize thegroup velocity of the incident light with that of the outgoing light(converted light) is meant selection of the angle of a nonlinear opticalcrystal relative to the optical path, polarized light direction of eachof the incident light and the outgoing light, temperature of the crystaland the like.

In the case of a LiNbO₃ crystal, for example, conventional quasi-phasematching, wherein a z plate is used, the polarized light directions ofboth the incident light and the outgoing light are set in parallel tothe z-axis and d₃₃ is utilized, fails to match the group velocity. Incontrast, according to the present invention, for example, quasi-phasematching is performed, wherein the polarized light direction of theincident light is made to be perpendicular to the z-axis and d₃₁ isutilized. As a result, the polarized light direction of the outgoinglight becomes parallel to the z-axis and the group velocity matching ofthe two is achieved while achieving the quasi-phase matching.

In addition, by employing a waveguide structure, and designing thewavelength dispersion of the waveguide, the matching with refractiveindex dispersion intrinsic to the material can be achieved, therebyrealizing the group velocity matching. For the waveguide, a knownstructure such as a proton exchange waveguide, a metal diffusionwaveguide such as Ti, Zn and the like, a ridge-type waveguide,dielectrics, a metal loading type waveguide and the like may be used.For example, a Ti diffusion waveguide, a Zn diffusion waveguide and aridge waveguide permit propagation of light having polarized lightdirections both perpendicular and parallel to the z-axis, and therefore,are waveguide structures suitable for the present invention. Moreover,by appropriately designing the shape of the waveguide, negative groupvelocity dispersion can be induced, thereby offsetting the groupvelocity dispersion of the material.

The wavelength conversion performed by the element is exemplified bysecond harmonic generation (SHG), optical parametric oscillation,optical parametric generation (including optical parametricamplification), difference frequency generation, sum-frequencygeneration and the like. Of the wavelength conversions, an operationreferred to as computing is exemplified by sampling and gating fortime-division multiplex communication, channel conversion and routingfor wavelength multiplex communication and the like, as mentioned above.

While the wavelength of the incident light is not limited, with theoptical fiber communication in view, 1.5 μm is a preferable wavelengthbecause a loss of optical fiber is small, dispersion of effectiverefractive index can be made nil, and the like.

When the incident light shows a pulse train, particularly when ashort-pulsed light having a pulse width of 1 ps or below, particularly300 fs or below, is used as the incident light, the utility of thepresent invention becomes particularly remarkable.

The nonlinear optical crystal usable for the element is preferablycapable of utilizing nonlinear optical constant d (particularlyoff-diagonal component) permitting group velocity matching when forminga poled structure. For example, typical ones such as LiNbO₃, LiTaO₃,X_(A)TiOX_(B)O₄ (X_(A)=K, Rb, Tl, CS, X_(B)=P, As) and the like, andthose obtained by doping these with various elements can be mentioned.Particularly, an MgO doped LiNbO₃ crystal is particularly preferablebecause it is superior in resistance to photorefractive damage, can actat room temperature, can produce large crystals in a large amount, canafford a crystal having high uniformity, and the like.

Of the off-diagonal components d_(ij) (i≠j) of the above-mentionednonlinear optical constant d, d₃₁ is the largest constant next to d₃₃and has a wide range of admissible phase matching and should beparticularly utilized.

To utilize the off-diagonal component d_(ij) (i≠j), particularly d₃₁, ofa nonlinear optical constant, the polarization reversal period isdetermined to satisfy the quasi-phase matching of wavelength conversionof the component.

For example, when forming an SHG element having an incident lightwavelength of 1.5 μm (central wavelength 1.55 μm), using MgO dopedLiNbO₃ as a nonlinear optical crystal, the polarization reversal periodonly needs to be set to 20 μm to utilize a nonlinear optical constantd₃₁ of the crystal. In this event, the polarized light direction of theoutgoing light (wavelength 0.78 μm) and the z direction of the crystalbecomes in a parallel relation, and the group velocity of the incidentlight (wavelength 1.5 μm) and the group velocity of the outgoing lightbecome identical. In other words, group velocity matching is achieved.The group velocity matched incident light and outgoing light arequasi-phase matched by the aforementioned polarization reversal period.

The element is useful for the wavelength conversion of a laser beam,particularly a pulse train, and can be applied to a light source foroptical communication utilizing an ultra-short pulse and an infraredbroad band light source having a wide wavelength band. Whether to form apoled structure only on the optical path of a nonlinear optical crystalor the entire element can be determined depending on the form, size,object and the like of the element.

EXAMPLES

In the Examples, an SHG element having an incident light wavelength of1.5 μm (central wavelength 1.55 μm) was actually formed using MgO dopedLiNbO₃ as a nonlinear optical crystal, a pulse train was projected andthe conversion efficiency was examined.

The polarization reversal period was set to 20 μm, the polarized lightdirection of the incident light and the outgoing light (SHG light: 0.78μm) was determined, and the nonlinear optical constant d₃₁ of MgO dopedLiNbO₃ was utilized to satisfy the group velocity matching conditionsand quasi-phase matching conditions of the incident light and theoutgoing light. The production step is shown in detail in the following.

As the crystal substrate material, a +z cut MgO doped LiNbO₃ crystalsubstrate processed into a single domain and having a thickness of 0.5mm and the entire length in the optical path direction (=element length)of 10 mm was used. The +Z-plane and −Z-plane of the crystal substratewere optically polished. A resist membrane having a periodic stripe maskpattern (grating pattern) was formed by photolithography on the +Z-planeof the crystal substrate. The stripe mask pattern is a stripe-likepattern having a strip-mask part and a strip-exposed part alternatelyarranged, wherein the width of the strip-mask part was 12 μm and thewidth of the strip-exposed part was 8 μm. A 4000 Å thick Au coat layerwas formed thereon by sputtering.

Then, a plus liquid electrode was contacted with the +Z-plane of thecrystal substrate, a minus liquid electrode was contacted with the−Z-plane, a polarization-reversal potential was applied to reverse thepolarization direction of the exposed part, thereby completing the poledstructure to create the element of the present invention.

Comparative Example

For comparison with the above-mentioned Example, as an SHG element bythe prior art, a wavelength conversion element having a poled structurewas formed in the same manner as in the above-mentioned Example exceptthat the polarization reversal period was set to 18.5 μm to utilize thenonlinear optical constant d₃₃.

Evaluation Test

A pulsed light having a wavelength of 1.5 μm and a pulse width of 300 fswas projected using an Er (erbium) doped fiber laser as anexciting-light source to the element obtained in the above-mentionedExample, the wavelength was converted and a pulsed light having a pulsewidth of 160 fs and a wavelength of 0.78 μm was allowed to go out. Thepolarized light direction of the incident light then was perpendicularto the z-axis of the element, and the polarized light direction of thelight output by the wavelength conversion was parallel to the z-axiselement, and the group velocity of both was the same. The outgoing lightwas due to wavelength conversion by quasi-phase matching using anonlinear optical constant d₃₁.

The outgoing light was observed. As a result, the pulse was notbroadened by the wavelength conversion and a conversion efficiency of20% was achieved.

A wavelength conversion element utilizing d₃₃ was subjected to a similarevaluation test as in the above-mentioned Example. The polarized lightdirection of the incident light and the outgoing light was parallel tothe z-axis of the element and the group velocity of the both was not thesame. The pulse shape of the outgoing light spread along the wavelengthconversion, and the conversion efficiency was as low as 1%.

INDUSTRIAL APPLICABILITY

As mentioned above, according to the present invention, the problemcaused by a difference in the group velocity between the incident lightand the outgoing light, which occurs during wavelength conversion of thepulsed light by quasi-phase matching has been resolved.

This application is based on a patent application No. 182394/2001 filedin Japan, the contents of which are hereby incorporated by reference.

The invention claimed is:
 1. An element for wavelength conversion and/oroptical computing, the element comprising: a nonlinear optical crystal;and a periodically-poled structure formed on the nonlinear opticalcrystal for converting an incident light into an outgoing light bywavelength conversion and/or optical computing, wherein group velocitymatching conditions synchronize a group velocity of the incident lightwith a group velocity of the outgoing light, and a polarization reversalperiod of the periodically-poled structure satisfies quasi-phasematching conditions for the wavelength conversion and/or opticalcomputing, the nonlinear optical crystal has an optical constant of d₃₁which is an off-diagonal component of d₃₁, tensor, the polarizationreversal period satisfies quasi-phase matching of the off-diagonalcomponent d₃₁, and the nonlinear optical crystal is adapted to receivethe incident light having a polarized light direction that isperpendicular to a z-axis of nonlinear optical crystal, the nonlinearoptical crystal is adapted to output the outgoing light having apolarized light direction that is parallel to the z-axis of thenonlinear optical crystal, and the group velocity of the incident lightand the group velocity of the outgoing light are the same to achievegroup velocity matching.
 2. The element of claim 1, wherein thenonlinear optical crystal is MgO doped LiNbO₃.
 3. The element of claim1, wherein the element is operable to perform the wavelength conversionand/or optical computing by second harmonic generation, opticalparametric oscillation, optical parametric generation, sum-frequencygeneration or difference frequency generation.
 4. The element of claim1, wherein the incident light is in a pulse train having a pulse widthof 1 ps or lower.